1. Field of the Invention
The present invention is directed, in general, to wireless communication systems and, more specifically, to a Single-Carrier Frequency Division Multiple Access (SC-FDMA) communication system and is further considered in the development of the 3rd Generation Partnership Project (3GPP) Evolved Universal Terrestrial Radio Access (E-UTRA) Long Term Evolution (LTE).
In particular, the present invention considers the transmission of positive or Negative ACKnowledgement signals (ACK or NAK, respectively) over multiple transmission time intervals in an SC-FDMA communication system.
2. Description of the Art
Several types of signals should be supported for the proper functionality of a communication system. In addition to data signals, which convey the information content, control signals also need to be transmitted from User Equipments (UEs) to their serving base station (or Node B) in the UpLink (UL) of the communication system and from the serving Node B to UEs in the DownLink (DL) of the communication system in order to enable proper processing of the data. A UE, also commonly referred to as terminal or mobile station, may be fixed or mobile and may be a wireless device, a cellular phone, a personal computer device, a wireless modem card, etc. A Node B is generally a fixed station and may also be called a Base Transceiver System (BTS), an access point, or some other terminology.
The acknowledgement signal, i.e., an ACK or NAK, also known as Hybrid Automatic Repeat reQuest (HARQ)-ACK, is a control signal associated with the application of HARQ and is in response to the data packet reception. A data packet is retransmitted if a NAK is received, and a new data packet may be transmitted if an ACK is received.
The transmission of signals carrying the data information from UEs is assumed to be through a Physical Uplink Shared CHannel (PUSCH). When there is no data, a UE transmits control signals through the Physical Uplink Control CHannel (PUCCH). When there is data, a UE transmits control signals through the PUSCH in order to maintain the single carrier property.
The UEs are assumed to transmit data or control signals over a Transmission Time Interval (TTI) corresponding to a sub-frame. FIG. 1 illustrates a block diagram of a sub-frame structure 110. The sub-frame includes two slots. Each slot 120 further includes seven symbols used for the transmission of data and/or control signals. Each symbol 130 further includes a Cyclic Prefix (CP) in order to mitigate interference due to channel propagation effects. The signal transmission in the first slot may be at the same part, or at a different part of the operating BandWidth (BW) than the signal transmission in the second slot. In addition to symbols carrying data or control signals, some other symbols may be used for the transmission of Reference Signals (RS), which are also referred to as pilots. The RS may be used for several receiver functions, including channel estimation and coherent demodulation of the received signal.
The transmission BW is assumed to include frequency resource units, which will be referred to herein as Resource Blocks (RBs). Herein, each RB is further assumed to include 12 sub-carriers and UEs can be allocated a multiple of P consecutive RBs 140 for PUSCH transmission and 1 RB for PUCCH transmission. Nevertheless, the above values are only illustrative and not restrictive to the embodiments of the invention.
FIG. 2 illustrates a PUCCH structure 210 for the ACK/NAK transmission in one slot of a sub-frame. The transmission in the other slot, which may be at a different part of the operating BW for frequency diversity, is assumed to have the same structure.
The PUCCH ACK/NAK transmission structure 210 includes the transmission of ACK/NAK signals and RS. The RS can be used for the coherent demodulation of the ACK/NAK signals. The ACK/NAK bits 220 modulate 230 a Constant Amplitude Zero Auto-Correlation (CAZAC) sequence 240, for example, with Binary Phase Shift Keying (BPSK) or Quadrature Phase Shift Keying (QPSK) modulation, which is then transmitted by the UE after performing the Inverse Fast Fourier Transform (IFFT) operation as will subsequently described. It is assumed that each RS 250 is transmitted through the same, unmodulated CAZAC sequence.
An example of CAZAC sequences is given by the following Equation (1):
                                          c            k                    ⁡                      (            n            )                          =                  exp          ⁡                      [                                                            j                  ⁢                                                                          ⁢                  2                  ⁢                  π                  ⁢                                                                          ⁢                  k                                L                            ⁢                              (                                  n                  +                                      n                    ⁢                                                                                                                                            ⁢                                                  n                          +                          1                                                                    2                                                                      )                                      ]                                              (        1        )            
In Equation (1), L is a length of the CAZAC sequence, n is an index of an element of the sequence n={0, 1, 2, . . . , L−1}, and k is an index of the sequence itself. For a given length L, if L is prime, there are L−1 distinct sequences. Therefore, the entire family of sequences is defined as k ranges in {1, 2, . . . , L−1}. However, it should be noted that the CAZAC sequences used for the ACK/NAK and RS transmission need not be generated using the exact above expression as will be further discussed below.
For CAZAC sequences of prime length L, the number of sequences is L−1. As the RBs are assumed to include an even number of sub-carriers, with 1 RB including 12 sub-carriers, the sequences used to transmit the ACK/NAK and RS can be generated, in the frequency or time domain, by either truncating a longer prime length (such as length 13) CAZAC sequence or by extending a shorter prime length (such as length 11) CAZAC sequence by repeating its first element(s) at the end (cyclic extension), although the resulting sequences do not fulfill the technical definition of a CAZAC sequence. Alternatively, CAZAC sequences with even length can be directly generated through a computer search for sequences satisfying the CAZAC properties.
FIG. 3 illustrates a transmitter structure for a CAZAC-based sequence that can be used either as an RS or to carry the ACK/NAK information bits after being modulated by them using BPSK (1 ACK/NAK bit) or QPSK (2 ACK/NAK bits) modulation, as illustrated in FIG. 2. The CAZAC sequence 310 is generated through one of the previously described methods, e.g., modulated for transmission of ACK/NAK bits, un-modulated for RS transmission. Thereafter, it is cyclically shifted 320 as will be subsequently described. The Discrete Fourier Transform (DFT) of the resulting sequence is then obtained 330. The sub-carriers 340 corresponding to the assigned transmission BW are selected 350, and the IFFT is performed 360. Finally, the Cyclic Prefix (CP) 370 and filtering 380 are applied to the transmitted signal. Zero padding is assumed to be inserted by the reference UE in sub-carriers used for the signal transmission by another UE and in guard sub-carriers (not shown). Moreover, for brevity, additional transmitter circuitry such as digital-to-analog converter, analog filters, amplifiers, transmitter antennas, etc., are not illustrated in FIG. 3.
At the receiver, the inverse (complementary) transmitter functions are performed. This is conceptually illustrated in FIG. 4, in which the reverse operations of those in FIG. 3 apply. As it is known in the art (not shown for brevity), an antenna receives the Radio-Frequency (RF) analog signal and, after passing further processing units, such as filters, amplifiers, frequency down-converters, and analog-to-digital converters, the digital received signal 410 passes through a time windowing unit 420 and the CP is removed 430. Subsequently, the receiver unit applies a Fast Fourier Transform (FFT 440), selects 450 the sub-carriers 460 used by the transmitter, applies an Inverse DFT (IDFT) 470, de-multiplexes (in time) the RS or ACK/NAK signal 480, and after obtaining a channel estimate based on the RS (not shown), it extracts the ACK/NAK bits 490. As for the transmitter, well known in the art receiver functionalities such as channel estimation and demodulation are not shown for brevity.
FIG. 5 illustrates an alternative generation method for the transmitted CAZAC sequence is in the frequency domain. The generation of the transmitted CAZAC sequence follows the same steps as in the time domain with two exceptions. The frequency domain version of the CAZAC sequence is used 510, i.e., the DFT of the CAZAC sequence is pre-computed and not included in the transmission chain, and the Cyclic Shift (CS) 550 is applied after the IFFT 540. The selection 520 of the sub-carriers 530 corresponding to the assigned transmission BW, and the application of CP 560 and filtering 570 to the transmitted signal 580, as well as other conventional functionalities (not shown), are as previously described for FIG. 3.
In FIG. 6, the reverse functions are again performed for the reception of the CAZAC-based sequence transmitted as in FIG. 5. The received signal 610 passes through a time windowing unit 620 and the CP is removed 630. Subsequently, the CS is restored 640, an FFT 650 is applied, and the transmitted sub-carriers 660 are selected 665. FIG. 6 also illustrates the subsequent correlation 670 with the replica 680 of the CAZAC-based sequence. Finally, the output 690 is obtained, which can then be passed to a channel estimation unit, such as a time-frequency interpolator, in case of an RS, or can be used to detect the transmitted information when the CAZAC-based sequence is modulated by the ACK/NAK information bits.
Different CSs of the same CAZAC sequence provide orthogonal CAZAC sequences. Therefore, different CSs of the same CAZAC sequence can be allocated to different UEs in the same RB for their RS or ACK/NAK transmission and achieve orthogonal UE multiplexing. This principle is illustrated in FIG. 7.
Referring to FIG. 7, in order for the multiple CAZAC sequences 710, 730, 750, 770 generated correspondingly from multiple CSs 720, 740, 760, 780 of the same root CAZAC sequence to be orthogonal, the CS value 790 should exceed the channel propagation delay spread D (including a time uncertainty error and filter spillover effects). If TS is the symbol duration, the number of CSs is equal to the mathematical floor of the ratio TS/D. For symbol duration of about 66 microseconds (14 symbols in a 1 millisecond sub-frame), a time separation of about 5.5 microseconds between consecutive CSs results in 12 CS values. If better protection against multipath propagation is needed, only every other (6 of the 12) CS may be used to provide time separation of about 11 microseconds.
Orthogonal multiplexing for the signals from different UEs in the same RB can be achieved not only through different CS values of the CAZAC sequence, as described in FIG. 7, but also through the use of different orthogonal time covers. The ACK/NAK and RS symbols are respectively multiplied with a first and a second orthogonal code. FIG. 8, which is identical to FIG. 2 with the exception of the inclusion of orthogonal time covering, further illustrates this concept.
Referring to FIG. 8, for the ACK/NAK, the orthogonal code is a Walsh-Hadamard (WH) code of length 4 ({+1,−1,−0,−1} 810 is used). For the RS, the orthogonal code is a Fourier code {1, ejθ, ej2θ} with
  θ  ∈            {              0        ,                              2            ⁢            π                    3                ,                              4            ⁢            π                    3                    }        ⁢          (              θ        =                              2            ⁢            π                    3                    820 is used in FIG. 8) or any other orthogonal code of length 3. The PUCCH multiplexing capacity with the use of orthogonal time covering is increased by a factor of 3 as it is constrained by the smaller length orthogonal code of the RS.
At the receiver the only additional operation needed, relative to the ones described in FIG. 4 and FIG. 6, is the orthogonal time de-covering. For example, for the structure illustrated in FIG. 8, multiplication with the WH code of {+1,−1,+1,−1} and the Fourier code of {1, e−jθ, e−j2θ} with θ=2π/3, needs to be performed for the received ACK/NAK and RS symbols, respectively.
PUSCH transmissions can be scheduled by the Node B through an UL Scheduling Assignment (SA) using the Physical DL Control CHannel (PDCCH) or they can be preconfigured. Using the PDCCH, a PUSCH transmission from a UE may generally occur at any sub-frame the Node B scheduler decides. Such PUSCH scheduling is referred to as dynamic. To avoid excessive PDCCH overhead, some PUSCH transmissions may be preconfigured to occur periodically at predetermined parts of the operating BW until re-configured. Such PUSCH transmission scheduling is referred to as semi-persistent.
FIG. 9 illustrates the concept of Semi-Persistent Scheduling (SPS), which is applicable for both DL and UL. SPS is typically used for communication services having relatively small BW requirements per sub-frame but need to be provided for many UEs. One typical example of such services is Voice over Internet Protocol (VoIP) where initial packet transmissions 910 are periodic over predetermined time intervals 920. Due to the large number of UEs potentially transmitting VoIP packets in a sub-frame, dynamic scheduling through the PDCCH highly inefficient and SPS can be used instead.
The Node B transmits the data packets to scheduled UEs through the Physical Downlink Shared CHannel (PDSCH). Similarly to the PUSCH, the PDSCH can be shared during the same sub-frame by multiple UEs for their packet reception from the same serving Node B, with each UE using a different part of the operating BW in order for mutual interference to be avoided. PDSCH transmissions can also be scheduled by the Node B through the PDCCH (dynamic scheduling) or can be preconfigured (SPS).
As the UL communication is considered, a focus will be on the ACK/NAK signals transmitted by UEs in response to a PDSCH transmission. Because PDSCH scheduling can be dynamic or semi-persistent, the transmission of ACK/NAK signals is respectively dynamic or semi-persistent (periodic). Also, because the periodic ACK/NAK transmissions are predetermined to occur at specific sub-frames, the respective resources (RB, CAZAC sequence CS, orthogonal code) can be also predetermined and semi-persistently assigned to a SPS UE. For dynamic ACK/NAK transmissions, no such pre-assignment is possible and the respective resources need to be dynamically determined in every sub-frame.
Several methods exist for a UE to use for mapping of the resources for its dynamic ACK/NAK transmission. For example, the DL SA may contain a few bits explicitly indicating these resources. Alternatively, implicit mapping based on the PDCCH resources used for the DL SA transmission may apply. The invention will be described using the latter option.
A DL SA includes Control Channel Elements (CCEs). The coding rate applied to the DL SA transmission to a UE depends on the received Signal to Interference and Noise Ratio (SINR) that UE experiences. For example, a high or low coding rate may respectively apply to the DL SA for a UE experiencing a high or low SINR. As the contents of the DL SA are fixed, different coding rates result to different number of CCEs. A DL SA with high coding rate, such as 2/3, may require 1 CCE for its transmission while a DL SA with low coding rate, such as 1/6, may require 4 CCEs for its transmission. It is assumed that the UL resources for the subsequent ACK/NAK transmission are derived by the number of the lowest CCE of the respective DL SA.
FIG. 10 further illustrates the concept of mapping the UL ACK/NAK resources to the lowest CCE number used for the previous DL SA transmission to a reference UE. The DL SA 1 1010 to UE 1 is mapped to 4 CCEs 1011, 1012, 1013, and 1014, the DL SA 2 1020 to UE 2 is mapped to 2 CCEs 1021 and 1022, and the DL SA 3 1030 to UE 3 is mapped to 1 CCE 1031. The resources for the subsequent UL ACK/NAK transmission are determined from the lowest CCE of the respective DL SAs and UE 1 uses resource ACK/NAK (A/N) 1 1040, UE 2 uses resource A/N 5 1044, and UE 3 uses resource A/N 7 1046. The resources A/N 2 1041, A/N 3 1042, A/N 4 1043, and A/N 6 1045 are not used for any dynamic ACK/NAK transmission. The transmission of UL SAs may also be based on the concept of CCE but this is not shown for brevity.
In addition to periodic and dynamic ACK/NAK signals, another periodic control signal a UE may transmit is the Channel Quality Indicator (CQI) informing the serving Node B of the channel conditions the UE experiences in the DL of the communication system, which are typically represented by a SINR. Other periodic control signals include the Service Request (SR) indicating a scheduling need, or the Rank Indicator (RI) indicating support for spatial multiplexing in case the serving Node B has 2 or more transmitter antennas. Therefore, the UL is assumed to support dynamic and semi-persistent PUSCH transmissions, dynamic ACK/NAK transmissions, periodic ACK/NAK transmissions, and other periodic control signals. All control channels are jointly referred to as PUCCH.
ACK/NAK signaling is the fundamental mechanism for a UE and its serving Node B to exchange information about the reception outcome of a prior data packet transmission. Therefore, the ACK/NAK reception reliability, as typically measured by the Bit Error Rate (BER), is essential to the proper operation of the communication system. For example, incorrect interpretation of NAK as ACK causes an incorrectly received packet to not be retransmitted, which in turn may result in a failure of the remaining communication session until the error is corrected by higher layers.
As several UEs may operate under low UL SINRs or be situated in coverage limited locations, the nominal ACK/NAK transmission over one sub-frame may often not be adequate to provide the required reception reliability. For such UEs, it is essential to extend their ACK/NAK transmission periods. A longer transmission period offers more ACK/NAK symbols which can be combined at the Node B receiver to effectively increase the total received SINR.